Through air dryers are well known expedients for drying wet paper webs, particularly tissue webs, conventionally employing a through air dryer roll for supporting and transporting the wet paper web and a through air dryer hood cooperable with the through air dryer roll to cause the passage of hot air through the through air dryer roll and the wet paper web thereon to dry the wet paper web.
Through air dryers employed in paper manufacture suffer from a number of deficiencies. Such arrangements typically employ bulky duct work. This results in high capital costs, fan pressure loss and the requirement that a considerable amount of physical space must be utilized. Conventional through air dryer fans are themselves bulky and difficult to locate close to paper machines.
The apparatus disclosed in above-noted co-pending U.S. patent application Ser. No. 08/707,798 eliminates the requirement for a bulky off machine fan and duct runs are either eliminated altogether or drastically reduced. When employing the apparatus there is less pressure across the seal between the rotating roll and stationary duct work as compared to prior art arrangements.
The apparatus of U.S. patent application Ser. No. 08/707,798, which is applicable for drying paper webs and other types of webs, includes a through air dryer roll for supporting a wet web while the wet web is being dried. The through air dryer roll has a double-ended cylindrical through air dryer roll wall with openings therein and defines a through air dryer roll interior communicating with the exterior of the through air dryer roll through the openings.
Through air dryer roll rotating means is provided for rotating the through air dryer roll.
A rotatable fan member having fan blades is located within the through air dryer roll interior and accommodated by an end of the through air dryer roll.
Fan member rotating means is provided for rotating the fan member within the through air dryer roll interior during rotation of the through air dryer roll by the through air dryer roll rotating means.
Air flow passageway defining means defines an air flow passageway between the fan member and the exterior of the through air dryer roll. The fan member upon rotation thereof by the fan member rotating means causes air flow through a web on the through air dryer roll through the openings of said through air dryer roll into the through air dryer roll interior, through the end of the through air dryer roll accommodating the fan member, and through the air flow passageway to the exterior of the through air dryer roll.
The air flow passageway defining means includes plenum defining means cooperable with the through air dryer roll to form a plenum located at the end of the through air dryer roll accommodating the fan member for receiving flowing air from the rotating fan member.
U.S. Pat. No. 5,416,979, issued May 23, 1995, discloses apparatus for drying a wet paper web during manufacture of the paper web. The apparatus allows the moisture profile of the web to be carefully controlled and adjusted. The apparatus includes a rotatable yankee dryer drum and a hood partially encompassing the rotatable dryer drum. A plurality of elongated heater nozzle boxes are disposed in the hood interior and extend across the dryer drum in the cross-machine direction, the elongated heater nozzle boxes being arrayed side-by-side in the machine direction. A plurality of gas burners is located in each of the nozzle box interiors and arranged side-by-side along the length of the nozzle box interiors for producing hot combustion gases within the nozzle box interiors of the heater nozzle boxes.
U.S. Pat. No. 5,425,852, issued Jun. 20, 1995, discloses a system for reducing blistering of a wet paper web on a yankee dryer. Blistering is controlled by directing compressed dry air toward the wet paper web in the space between a pressure roll external of the yankee dryer hood and the entry opening between the dryer hood and the yankee dryer drum. The flow of compressed air is controlled so that air flow can be modified across the full width of the wet paper web.
U.S. Pat. No. 5,465,504, issued Nov. 14, 1995, discloses drying a paper web on a yankee dryer. The moisture profile is controlled by arraying a plurality of gas burners in a cross-machine direction within the yankee dryer hood to direct heat toward incremental width portions of the paper web.
The invention disclosed in above-noted co-pending U.S. patent application Ser. No. 08/651,890 relates to drying a wet web and modifying the moisture profile thereof. In the embodiment disclosed in this latter application, the invention is employed to modify the moisture profile of a wet paper web while the paper web is being manufactured in a system utilizing a through air dryer; however, the principles of the invention are applicable to drying other types of webs. As noted above, through air dryers are well known expedients for drying wet paper webs, particularly tissue webs, conventionally employing a through air dryer roll for supporting and transporting the wet paper web and a through air dryer hood cooperable with the through air dryer roll to cause the passage of hot air through the through air dryer roll and the wet paper web thereon to dry the wet paper web. Flat bed through air dryers not employing a through air dryer roll and instead employing other forms of web support structure are also known.
Through air dryers are shown, for example, in the following United States patents: U.S. Pat. No. 3,303,576, issued Feb. 14, 1967, U.S. Pat. No. 3,432,936, issued Mar. 18, 1969, and U.S. Pat. No. 3,821,068, issued Jun. 28, 1974.
It is, of course, highly desirable to maintain a uniform moisture profile in a paper web during manufacture thereof in order to attain a high quality end product of uniform character. Conventional through air dryer systems present major problems with respect to obtaining and maintaining web uniform moisture profiles.
The problem is that through air dryers rely upon the passage of heated air through the wet paper web to accomplish drying. Any differences in the moisture profile of the wet paper web are actually amplified during the through drying process since web permeability is higher in the drier areas of the web and these areas will tend to dry faster than the wetter, less permeable portions of the web. In other words, the drier portions of the web tend to get even drier relative to the wetter portions. In order to overcome this problem it has been the practice in the prior art to "over dry" the entire web in order to obtain a degree of uniform dryness. This, of course, is highly wasteful of energy.
Conventional through air dryer arrangements have other drawbacks as well. Typically, conventional through air dryers require large volume combustion and air mixing chambers aimed at uniformly distributing heat. These arrangements take up considerable physical space and are capital intensive. Through air dryers conventionally employ structures within the hood interior, such as turning vanes, which are for the purpose of receiving heated air from a separate combustion chamber and distributing it uniformly prior to engagement with the wet paper web being dried in an attempt to maintain a uniform moisture profile. These mechanisms are not only complicated and expensive but are not as effective as desired from the stand point of maintaining a level moisture profile. Fan pressure loss and requirement for bulky and extensive duct work are additional problems in the prior art.
The invention of U.S. patent application Ser. No. 08/651,890 relates to an apparatus and to a method which are highly effective in controlling the moisture profile of a wet web being through air dried. The invention, as disclosed in such application, is employed to dry a wet paper web; however, the principles of the invention are applicable to dry other types of webs. The invention allows the elimination of the large combustion and air mixing chambers typically employed in prior art through air dry arrangements. Consequently, the apparatus constructed in accordance with the teachings of the invention occupies less physical space and requires a lower capital expenditure than typical prior art systems. Fan pressure loss commonly associated with prior art arrangements is minimized. Temperature and air flow distribution in the through air drier hood interior is less critical with the arrangement of the invention under discussion as compared to prior art arrangements because a profiling burner incorporated in the apparatus can readily compensate for variations. A considerable improvement in product quality results due to the fact that the apparatus and method of the invention provide a readily attained level moisture profile and the product need not be over dried.
The burner is much closer to the web when it is located in the hood than conventional. The volume of heated air between the burner and the web is small compared to conventional. This allows the hot supply air to cool down faster in the event the web is interrupted. This is important in tissue applications because the wet web protects a supporting fabric which cannot tolerate the hot supply air temperature.
The apparatus disclosed in U.S. patent application Ser. No. 08/651,890 is for the purpose of drying a wet web, such as a wet paper web, and modifying the moisture profile of the wet web. The apparatus includes a through air dryer hood defining a hood interior and at least one air supply inlet communicating with the hood interior.
A through air dryer support, such as a through air dryer roll, is mounted for movement relative to the through air dryer hood, the through air dryer web support for engaging a wet web as the web moves in a machine direction in communication with the hood interior.
Heater means is located within the hood interior for heating air within the hood interior during movement of the air from the air supply inlet to the through air dryer web support for drying the wet web.
The heater means comprises a plurality of gas burners located within the hood interior. The gas burners are disposed alongside one another in the cross-machine direction with each gas burner extending only part way along the length of the through air dryer support.
The apparatus additionally comprises air flow separator means located between the gas burners and the through air dryer roll for dividing air flowing through the hood interior between the gas burners and the through air dryer support into separate air flow portions. Each of the air flow portions is directed toward an incremental width portion of the wet web engaging the through air dryer web support.
In addition to the patents noted above, the following United States patents are considered representative of the current state of the prior art: U.S. Pat. No. 5,397,437, issued Mar. 14, 1995, U.S. Pat. No. 5,174,046, issued Dec. 29, 1992, U.S. Pat. No. 5,150,535, issued Sep. 29, 1992, U.S. Pat. No. 4,654,981, issued Apr. 7, 1987, U.S. Pat. No. 4,462,868, issued Jul. 31, 1984, U.S. Pat. No. 3,919,783, issued Nov. 18, 1975, U.S. Pat. No. 3,891,500, issued Jun. 24, 1975, U.S. Pat. No. 3,541,697, issued Nov. 24, 1970, and U.S. Pat. No. 3,340,617, issued Sep. 12, 1967.